Orange: made Compositing more interactive. It now has an event based

system tracking changes in nodes, making sure only these nodes and
the ones that depend, are executed.

Further the 'time cursor' now counts down to indicate which node is being
done.

Also: you now can disable the "use nodes" button in the header, edit all
changes, and when you press that button again it nicely executes the
changes.

Still on the todo:
- make compositing threaded
- find a way to nicely exit compositing on input events... so the UI
  keeps being responsive
- idea; a 'percentage' menu in header to enforce calculations on smaller
  images temporally

source/blender/src/header_node.c

@@ -82,7 +82,11 @@ void do_node_buttons(ScrArea *sa, unsigned short event)
 			break;
 			
 		case B_NODE_USESCENE:
-			node_composit_default(G.scene);
+			if(G.scene->use_nodes) {
+				if(G.scene->nodetree==NULL)
+					node_composit_default(G.scene);
+				addqueue(curarea->win, UI_BUT_EVENT, B_NODE_TREE_EXEC);
+			}
 			snode_set_context(snode);
 			allqueue(REDRAWNODE, 0);
 			break;